The present invention is directed to a light emitting diode (LED) light engine and a method of manufacturing the light engine.
In a conventional method of making an LED light engine that emits light of a color other than a color emitted by the LED, a phosphor is used to convert the light emitted from the LED to produce the desired emission color. The phosphor is selected depending on the wavelengths emitted by the LED and the overall color of the light to be emitted by the light engine.
For example, in an LED light engine that emits white light, a blue LED is embedded in or covered with a lens or case (the LED optic) made of a high index of refraction transparent polymer, such as silicone. A phosphor that converts the blue light to yellow light (for example, a YAG:Ce phosphor) may be mixed with the polymer to provide volumetric blue light conversion. The yellow light emitted by the phosphor combines with the residual unconverted blue light from the LED to produce an overall white emission from the LED light engine.
A portion of the light going through the phosphor undergoes a Stokes shift as it is converted from one wavelength range to another wavelength range. Thus, phosphor-based LEDs have a lower efficiency than normal LEDs due to the heat loss from the Stokes shift. Moreover, the proximity of the phosphor to the LED die leads to lumen degradation of the package due to the heat produced by the die and by the Stokes shift. Nevertheless, the phosphor method is a popular technique for manufacturing white LEDs.
Accordingly, LED light engines, particularly those that produce white light, require thoughtful thermal design.